Catalyst with a base of modified MFI zeolite, and its use in the isomerization of a C8 aromatic cut

ABSTRACT

The invention is concerned with a catalyst comprising a matrix, a MFI type zeolite and at least one group VIII element, in which the crystals of the MFI type zeolite are desaluminated on their outer surface by at least one treatment with at least one solution of a fluorosilicate of a cation selected from the group formed by NH 4   + , alkylammoniums, K + , Na + , Li + , Ba 2+ , Mg 2+ , Cd 2+ , Cu + , Cu 2+ , Ca 2+ , Cs + , Fe 2+ , Co 2+ , Pb 2+ , Mn 2+ , Rb + , Ag + , Sr 2+ , Zn 2+ , Tl +   and H + , the global desalumination rate of said MFI zeolite thus treated being less than 5 atomic percent.

This appln is a continuation of Ser. No. 08/451,149, filed May 26, 1995,now abandoned, which is a divisional of application Ser. No. 08/187,026,filed Jan. 27, 1994.

BACKGROUND OF THE INVENTION

The present invention is concerned with an alumino-silicate typecatalyst comprising an MFI zeolite, the selectivity and/or catalyticproperties of which has/have been modified by desalumination of theouter surface of its crystals, and comprising at least one group VIIImetal from the periodic classification of elements (Handbook ofChemistry and Physics, 65th edition, 1984-85), and a matrix. Saidcatalyst is used in isomerization reactions of C₈ aromatic hydrocarbons.The invention is also concerned with the process for the preparation ofsaid catalyst. Usually, the outer surface of the zeolite crystals ismodified by at least one treatment of the MFI by at least one solutionof a fluorosilicic acid salt. Preferably, the salt selected does notresult in the formation of aluminium salts which are not soluble inwater.

At the present time, the catalysts used industrially in isomerisationreactions of C₈ aromatic cuts mainly have a base of ZSM-5 zeolite of MFIstructural type, alone or mixed with other zeolites. These catalysts aredescribed, in particular, in the patents U.S. Pat. No. 3,856,872, U.S.Pat. No. 4,098,836, U.S. Pat. No. 4,159,282, U.S. Pat. No. 4,467,129,U.S. Pat. No. 4,428,773, European Patent 138,617 and U.S. Pat. No.5,028,573.

The ZSM-5 zeolite is interesting because of its selectivity of shapewhich gives an advantageous selectivity with respect to para-xylene andalso in its selectivity with respect to undesirable secondarydismutation reactions which remain at a lower level than those recordedon other zeolites with larger pore openings. In fact, 12MR zeolites withlarger pore openings (opening with 12 oxygen atoms) have also been used,such as mordenite. The catalysts with a base of mordenite are described,in particular, in the patents U.S. Pat. No. 4,723,051, U.S. Pat. No.4,665,258, and French Patent 2,477,903. However, these zeolites do nothave special properties of geometric selectivity. This means that,irrespective of their Si/Al ratio, their selectivities with respect topara-xylene are lower than those obtained for ZSM-5 zeolite, and, inparticular, there is a very great amount of trimethylbenzenes produced.The production of trimethylbenzenes by dismutation is actually promotedin mordenite whose microporous system is more open than that of ZSM-5:the openings are with 12 oxygens instead of 10 for ZSM-5.

SUMMARY OF THE INVENTION

The Applicant has surprisingly discovered that by carrying out at leastone dealumination treatment of a MFI by using at least one fluorosilicicsolution of a cation, and, in particular, at least one solution ofammonium hexafluorosilicate, it is possible to obtain active andselective catalysts for the isomerization reaction of C₈ aromatics. Anoverall or "global" dealumination rate is thus obtained which is lessthan 5% ion an atomic basis, i.e., for MFI zeolite thus treated, that isto say that the dealumination treatment draws off a maximum of 5% of thealuminium atoms present in the zeolite framework. The outer surface ofthe zeolite is dealuminated at least 37%, i.e., 37-61%, or 52-61%. Saidtreatment imparts to the MFI thus treated greatly improved selectivityproperties. This is to be seen by a surprising inhibition of undesirablesecondary reactions, such as the dismutation reaction, and by anincrease in selectivity with respect to para-xylene. The new modifiedMFI also gives selectivities with respect to desalkylation reactionswhich are equivalent to those of catalysts with a base of untreated MFI.The solids thus obtained perform better than prior art MFI solids withisomerization of C₈ aromatics.

The MFI used in the catalyst of the present invention is prepared from aMFI which is synthesized just as well in hydroxide medium (U.S. Pat. No.3,702,886) as in fluoride, in the presence of an organic structurer (forexample, U.S. Pat. No. 5,010,048 and Patent Applications European Patent342,075, European 469,151 and European Patent 472,462) or in the absenceof an organic structurer (for example, European Application 500,413).

The MFI to which at least one treatment is given with at least onesolution of fluorosilicic acid salts is preferably obtained directly onsynthesis with the desired Si/Al atomic ratio, or is obtained bydealumination of a MFI which has a lower Si/Al ratio.

The MFI selectivity is modified by at least one treatment with at leastone solution of fluorosilicate for MFI with a Si/Al atomic ratio whichis between about 5 and 1000, preferably between 5 and 500, and stillmore preferably between 5 and 250. Said MFI are in the form of Na⁺, H⁺,NH₄ ⁺, or they contain the organic structurer which was used in theirsynthesis, or they are all in the form of mixed combinations of the fourforms cited hereinabove, preferably in the form of NH₄ ⁺ and/or the formcontaining the organic structurer.

When the treatment with at least one fluorosilicate solution is carriedout on NH4⁺ forms of MFI, the following methods are used for placementin NH₄ ⁺ form, depending on the case at hand:

a) The MFI has been synthesized in the absence of organic structurer. Inthis case, the crude MFI from the synthesis operation contains sodium inits microporosity which is removed by several ionic exchanges withconcentrated solutions of ammonium nitrate (10N) which allows a contentof sodium by weight to be obtained in relation to weight of dry MFIwhich is usually less than 2000 ppm, preferably less than 1000 ppm, and,more preferably still, less than 500 ppm.

b) The MFI has been synthesised in the presence of an organicstructurer. In this case the crude MFI from the synthesis operationcontains the organic structurer and sodium in its microporosity. Thecrude MFI from the synthesis operation is first of all subjected to acalcination operation in air in order to remove from its structurepractically all the organic structurer which is found there. Then, it issubjected to a plurality of ionic exchanges by concentrated solutions ofammonium nitrate (10N) which enable a content by weight of sodium to beobtained in relation to the weight of dry MFI which is usually less than2000 ppm, preferably less than 1000 ppm, and still more preferably lessthan 500 ppm.

When the treatment with at least one fluorosilicate solution is carriedout directly on the MFI forms containing the organic structurer noinitial treatment is carried out on the MFI. The residual sodiumcontained in the microporosity of the zeolite is thus practicallyeliminated, after said treatment, by carrying out three ionic exchangeswith solutions of ammonium nitrate.

The fluorosilicate used as the dealumination agent and as the siliconsource which enable silicon atoms to be reinserted into the crystalnetwork of the MFI to replace the aluminium atoms removed can be one ofthe salts of the following formula: M_(2/x) SiF₆ where M is a metalcation or non-metallic cation with the valence x. The M cations can thusbe NH₄ ⁺, an alkylammonium, K⁺, Na⁺, Li⁺, Ba²⁺, Mg²⁺, Cd²⁺, Cu⁺, Cu²⁺,Ca²⁺, Cs⁺, Fe²⁺, Co²⁺, Pb²⁺, Mn²⁺, Rb⁺, Ag⁺, Sr²⁺, Zn²⁺, Tl⁺ and H⁺.Preferably, ammonium hexafluorosilicate is used because it results inthe formation of aluminium salt (NH₄)₃ AlF₆ which is soluble in waterwhich can be easily eliminated. Usually, the treatment temperature isbetween 20 and 100° C., and preferably between 50 and 100° C. Thetreatment of the MFI is carried out in the presence of ammonium acetatewhich enables the pH of the reaction medium to be fixed at values ofbetween 4 and 8, preferably between 5.5 and 7, these being pH values forwhich the zeolite does not suffer any destruction to the framework bydirect acid attack.

After the step where fluorosilicate solution is added to the MFIsuspension in an ammonium acetate solution, the reaction mixture isleft, with vigorous agitation, at the desired temperature for a periodof between 30 minutes and 48 hours, but preferably between 1 and 5hours.

The MFI is then filtered at the reaction temperature and is washedthoroughly in boiling water. The volume of boiling water used for thesewashing operations corresponds to a v/p=150 ml/g, (the v/p ratio is theratio of the volume of boiling water to the amount of dry zeolitetreated. The MFI type zeolite is dried in a flow of air at 450° C. for 4hours).

After this (these) treatment(s), the modified MFI is subjected to a heattreatment which is intended either to decompose the ammonium cationspresent within the network and to thus obtain the acid form (H-M) of theMFI if the treatment has been carried out on the NH₄ ⁺ form of the MFI,or it is subjected to a calcination operation in air to remove theorganic structurer, followed by a plurality of ionic exchanges withconcentrated solutions of ammonium nitrate (10N) which enable a contentby weight of sodium to be obtained in relation to the dry MFI which isusually less than 2000 ppm, preferably less than 1000 ppm, and, stillmore preferably, less than 500 ppm in instances where the treatment iscarried out on the MFI containing the organic structurer which has beenused for its synthesis.

The zeolite can then be subjected to the deposit of at least one groupVIII metal, preferably selected from the group formed by platinum andpalladium, and shaped using any technique known to the skilled person.It can, in particular, be mixed with a matrix, usually amorphous, forexample with a damp powder of alumina gel. The mixture is then shaped,by extrusion, for example, through a drawplate. The content of MFI inthe mixture thus obtained is usually between 0.5 and 99.99% andadvantageously between 40 and 90% by weight in relation to the mixture(MFI+matrix). It is more particularly between about 10 and 60%, and,preferably, between about 15 and 40% by weight in relation to themixture (MFI+matrix).

In that which follows, the term, "support" will be used to denote themixture of MFI+matrix.

The shaping operation can be carried out with matrices other thanalumina, such as magnesium, silica alumina, natural clays (kaolin,bentonite) and by way of other techniques than extrusion techniques,such as the formation of pastilles or dragees.

The hydrogenizing group VIII metal, preferably Pt and/or Pd, can also bedeposited on the support using any process known to the skilled personenabling the metal to be deposited on the MFI. It is possible to use thecationic exchange technique competitively where the competitor ispreferably ammonium nitrate, the competition ratio being at least equalto about 50 and advantageously about 50 to 200. With platinum orpalladium, a tetramine complex is usually used of platinum, or atetramine complex of palladium: these latter are then practically alldeposited over the MFI. This cationic exchange technique can also beused for directly depositing the metal on the MFI powder, before it iseventually mixed with a matrix.

Depositing of the group VIII metal (or metals) is usually followed by acalcination operation in air or oxygen, usually between 300 and 600° C.for 0.5 to 10 hours, preferably between 350° C. and 550° C. for 1 to 4hours. Reduction is then possible in hydrogen, usually at a temperatureof between 300 and 600° C. for 1 to 10 hours; preferably, the operationtakes place between 350° C. and 550° C. for 2 to 5 hours. The content ofgroup VIII metal (preferably Pt and/or Pd) deposited on the catalystwhich is obtained after the end of the exchange operation is usuallybetween 0.05 and 1.5%, preferably between 0.1 and 1% by weight inrelation to the catalyst as a whole.

It is also possible to deposit platinum and/or palladium no longerdirectly on the MFI, but on the aluminic binding agent, before or afterthe shaping operation, by carrying out anionic exchange withhexachloroplatinic acid, hexachloropalladic acid and/or palladiumchloride in the presence of a competitor agent, for example hydrochloricacid. Usually, after platinum and/or palladium has/have been deposited,the catalyst is subjected to a calcination operation, as before, andthen reduced in hydrogen, as stated hereinabove.

The bifunctional catalyst obtained by way of the above operations can beused particularly in isomerization reactions for an aromatic C₈ cutcomprising either a mixture of xylenes alone or a mixture of xylene(s)and ethylbenzene. The isomerization of alkyl-aromatics, and, inparticular, of xylenes, is very important commercially. Usually, it ispara-xylene, in particular, which is the most sought after productbecause it is used, in particular, as an intermediate item in themanufacture of polyester fibres. It is preferable to manufacturepara-xylene by isomerising meta-xylene, this latter being obtainable byisomerisation of ortho-xylene. Since it is difficult to separate ethylbenzene by distillation of the mixture of xylenes (the boiling points ofthe various compounds are very close), ethyl benzene is very often foundin the isomerisation charge of aromatic C₈ hydrocarbons.

The operating conditions of the isomerization process for an aromatic C₈cut which is carried out in the presence of at least one catalystaccording to the invention are as follows:

temperature between 240 and 600° C., preferably between 350 and 510° C.,

pressure between 0.05 and 10 MPa, preferably between 0.2 and 3 MPa,

spatial speed (pph, in mass of charge per unit of charge of catalyst andper hour), between 0.5 and 200 h⁻¹, preferably between 2 and 100 h⁻¹,

molar ratio of hydrogen to hydrocarbon of the charge (H₂ /HC) between0.5 and 12, preferably between 2 and 6.

The following examples illustrate the invention, without, however,limiting its scope; they are given either for a charge which is formedof 80% ortho-xylene and of 20% ethylbenzene (% by weight), examples 2,3, 5 and 6, or for a charge which is formed of 80% meta-xylene and 20%ethyl benzene (% by weight), examples 1 and 4.

EXAMPLES Example 1 Catalyst C1 According to the Invention

The raw material used is an MFI which is synthesized in a fluoridemedium which has a global atomic Si/Al ratio of 10.5 and a content byweight of sodium in relation to dry MF1 of about 7%.

The MFI is first of all subjected to 3 ionic exchanges in a solution of10N NH₄ NO₃ at about 100° C. for 4 hours for each exchange. The contentby weight of sodium is thus less than 50 ppm. The MFI form NH₄ ⁺ thenhas a global Si/Al ratio of 10.5, a Si/Al ratio measured by XPS (ESCA)characteristic of the outer surface of the MFI crystallites of 10, anelementary mesh volume of 5.393 nm³, a porous volume in nitrogen,measured at -196° C. and with P/Po=0.19 of 0.22 cm³ of liquid per gramof MFI and a specific surface measured using the B.E.T. method of 410 m²/g.

The MFI form NH₄ obtained hereinabove is then subjected to treatmentwith a solution of ammonium hexafluorosilicate. To that end, 20 grams ofdry zeolite is placed in suspension in 400 ml of a solution of ammoniumacetate (30 grams of ammonium acetate for 200 ml distilled water). Thissuspension is then placed in a 500 ml tricol provided with a reflux, andmechanical agitation means. The initial pH of the medium is 7.05. Thetemperature is brought to 80° C. Then, using a pump, 116 ml of asolution of 0.5 M ammonium hexafluorosilicate is introduced. Afteradding it the amount of ammonium hexafluorosilicate injected is 0.29mole for 100 grams dry zeolite. The system is kept at the reactiontemperature for another 2 hours. Then, the solution is cooled to ambienttemperature, and the measured value of the pH at the end of the reactionis 6.1. The solid is then filtered and washed with at least 6 liters ofboiling distilled water, that is to say with at least 300 ml distilledwater per gram of dry zeolite (V/P=300 ml/g). The zeolite thus treatedis dried in the test tube at 105° C. for one night, and then calcined inair in such a way as to remove the ammonia from the MFI and to obtainthe H⁺ form. The solid obtained after these treatments is called HMFI-1.

This latter is then thoroughly mixed with the alumina on which 0.3% byweight of platinum is dispersed. The catalyst formed by the HMFI-1mixture plus alumina contains 40% by weight of alumina. The content byweight of the platinum of the end catalyst (containing HMFI-1) is thusabout 0.12% by weight.

The C1 catalyst thus produced is then shaped by a pastille-formingoperation, calcined in air at 550° C. for 2 hours and reduced inhydrogen to 500° C. for 3 hours.

The catalyst C1 is then tested with isomerization of the meta-xylenemixture (80% by weight) and ethyl benzene (20% by weight) at atemperature of 410° C., at a pressure of 1.2 MPa and with a molar ratioof hydrogen to hydrocarbons (H₂ /HC) of about 4.

The performance of the catalyst C1 and of the catalysts prepared in thefollowing examples, given in Table 1, are defined by: ##EQU1##

    Conversion of meta-xylene (%)=\F(mass of meta-xylene in the charge-mass of meta-xylene in the product; mass of meta-xylene in the charge)×100

Example 2 Catalyst C2 According to the Invention

The raw material used is an MFI in H⁺ form which has a Si/Al atomicratio of 27, an elementary mesh volume of 5.341 nm³, a content of sodiumof less than 50 ppm by weight, a porous volume in nitrogen, measured at-196° C. and with P/Po=0.19 of 0.19 cm³ of liquid per gram of MFI and aspecific surface measured using the B.E.T. method of 439 m² /g.

The MFI is first of all subjected to 3 ionic exchanges in a solution of10N NH₄ NO₃ at about 100° C. for 4 hours, for each exchange, in such away as to obtain it in NH₄ ⁺ form. It thus has a Si/Al ratio measured byXPS (ESCA) which is characteristic of the outer surface of the MFIcrystallites of 26, almost identical to the global Si/Al ratio measuredby X fluorescence (FX).

The MPI form NH₄ ⁺ obtained hereinabove is then subjected to thetreatment with ammonium hexafluorosilicate. To that end, 20 grams of dryzeolite is placed in suspension in 200 ml of a solution of ammoniumacetate (20 grams of ammonium acetate for 200 ml distilled water). Thissuspension is then placed in a 500 ml tricol which is provided with areflux, and with mechanical agitation means. The initial pH of themedium is 7.0. The temperature is brought to 80° C. Then, with the aidof a pump, 80 ml of a 0.3 M ammonium hexafluorosilicate solution isintroduced. After it has been added, the amount of ammoniumhexafluorosilicate injected represents 0.12 moles for 100 grams of dryzeolite. The system is kept at the reaction temperature for another 2hours. Then, the solution is cooled to ambient temperature, and themeasured pH value at the end of the reaction is 5.5. The solid is thenfiltered and washed with at least 6 liters of boiling distilled water,that is to say at least 300 ml distilled water per gram of dry zeolite(V/P=300 ml/g). The zeolite thus treated is dried in the test tube at105° C. for one night, then calcined in dry air in such a way as toremove the ammonia from the MPI and to obtain the H⁺ form. The solidobtained at the end of these treatments is called HMFI-2.

The characteristics of the solid obtained (catalyst C2) are a globalatomic Si/Al ratio of 27 and a Si/Al ratio of the XPS surface of 403.The surface ratio is measured by XPS (ESCA) and is characteristic of theouter surface of the crystals of the MFI treated.

The steps for mixing the MFI and the alumina, for dispersing theplatinum, for shaping, reducing the catalyst and the catalysts and theconditions for the isomerization test are identical to those describedin Example 1.

The catalyst C2 is then tested with respect to isomerization of themixture of ortho-xylene (80% by weight) and ethylbenzene (20% byweight), at a temperature of 410° C., at a pressure of 1.2 MPa and witha molar ratio of hydrogen to hydrocarbons (H₂ /HC) of about 4.

The performances of the catalyst C2 thus obtained and according to theinvention (whose platinum content is about 0.12% by weight) are shown inTable 2.

Example 3 Catalyst C3 According to the Invention

The raw material used is a MFI containing the organic structurer whichhas been used for its synthesis and which has an atomic Si/Al ratio of45 (measured by X fluorescence) and an elementary mesh volume of 5.367nm³, a sodium content of 0.7% by weight.

The MFI described hereinabove is subjected to the treatment withammonium hexafluorosilicate. To that end, 20 grams of dry zeolite isplaced in suspension in 200 ml of a solution of ammonium acetate (20grams of ammonium acetate for 200 ml distilled water). This suspensionis then placed in a 500 ml tricol which is provided with a reflux, andwith mechanical agitation means. The initial pH of the medium is 6.8.The temperature is brought to 80° C. Then, with the aid of a pump, 55 mlof a solution of 0.4 M ammonium hexafluorosilicate is introduced. Afterit has been added, the amount of ammonium hexafluorosilicate injected is0.11 mole for 100 grams dry zeolite. The system is kept at the reactiontemperature for another 2 hours. Then, the solution is cooled to ambienttemperature, and the measured pH value at the end of the reaction is5.7. The solid is then filtered and washed with at least 6 liters ofboiling distilled water, that is to say at least 300 ml distilled waterper gram of dry zeolite (V/P=300 ml/g). The zeolite thus treated isdried in the test tube at 105° C. for one night, and then it is calcinedin dry air in such a way as to completely remove the organic structurercontained in its pore structure. Then, the MFI is subjected to 3 ionicexchanges in a solution of 10N NH₄ NO₃ at about 100° C. for 4 hours, foreach exchange, in order to obtain it in the NH₄ ⁺ form. Finally, the MFItreated and in NH₄ ⁺ is calcined in dry air in order to remove theammonia from it and to obtain the H⁺ form. The solid obtained afterthese treatments is called HMFI-3.

The steps for mixing the MFI and the alumina, for dispersing theplatinum, for shaping and for reducing the catalyst are identical tothose described in Example 1.

The characteristics of the solid obtained (catalyst C3) are a globalatomic Si/Al ratio of 45 and a Si/Al XPS surface ratio above 95. Thesurface ratio is measured by XPS (ESCA) and is characteristic of theouter surface of the crystals of the MFI treated.

The catalyst C3 is then tested with respect to isomerization of themixture of ortho-xylene (80% by weight) and ethylbenzene (20% by weight)at a temperature of 410° C., at a pressure of 1.2 MPa and with a molarratio of hydrogen to hydrocarbons (H₂ /HC) of about 4.

The performance of the catalyst C3 thus obtained and according to theinvention (whose platinum content is about 0.12%) are given in Table 2.

Example 4 NC1 Catalyst Not According to the Invention

The raw material used in this example is MFI in NH₄ ⁺ form, prepared inExample 1, but not treated according to the invention.

This latter is then thoroughly mixed with the alumina on which 0.3% byweight of platinum is dispersed. The catalyst constituted of the mixtureMFI+alumina contains 40% by weight of alumina. The content by weight ofplatinum of the end catalyst (containing MFI) is thus about 0.12% byweight.

The NC1 catalyst thus produced is then shaped by a pastille formingoperation, calcined in air at 550° C. for 2 hours and reduced inhydrogen at 500° C. for 3 hours.

The NC1 catalyst is then tested with isomerization of the mixture ofmeta-xylene (80% by weight) and ethylbenzene (20% by weight), at atemperature of 410° C., at a pressure of 1.2 MPa and with a molar ratioof hydrogen to hydrocarbons (H₂ /HC) of about 4.

The performance of the NC1 catalyst are given in Table 1.

Example 5 Catalyst NC2 Not According to the Invention

The raw material used in this example is the MFI in H⁺ form used inExample 2 and not treated according to the invention.

The steps for forming NH₄ ⁺ from the MFI, for mixing the MFI and thealumina, for dispersing the platinum, for shaping and reducing thecatalyst are identical to those described in Example 1.

The catalyst NC2 is then tested with respect to isomerization of themixture of ortho-xylene (80% by weight) and ethyl benzene (20% byweight) at a temperature of 410° C., at a pressure of 1.2 MPa and with amolar ratio of hydrogen to hydrocarbons (H₂ /HC) of about 4.

The performance of the catalyst NC2 thus obtained and not according tothe invention (whose platinum content is about 0.12%) are given in Table2.

Example 6 Catalyst NC3 Not According to the Invention

The raw material used in this example not according to the invention isa MFI containing the organic structurer which has been used for itssynthesis and which has an atomic Si/Al ratio of 45, but which is nottreated according to the invention.

The zeolite is dried in the test tube at 105° C. for one night, thencalcined in dry air in order to completely remove the organic structurercontained in its pore structure. The MFI is then subjected to 3 ionicexchanges in a solution of 10N NH₄ NO₃ at about 100° C. for 4 hours, foreach exchange operation in such a way as to obtain it in NH₄ ⁺ form.Finally, the MFI treated and in NH₄ ⁺ form is calcined in dry air inorder to remove the ammonia from it and to obtain the H⁺ form.

The steps for mixing the MFI and the alumina, for dispersing theplatinum, for shaping and for reducing the catalyst are identical tothose described in Example 1.

The catalyst NC3 is then tested with respect to isomerization of themixture of ortho-xylene (80% by weight) and ethylbenzene (20% by weight)at a temperature of 410° C., at a pressure of 1.2 MPa and with a molarratio of hydrogen to hydrocarbons (H₂ /HC) of about 4.

The performance of the catalyst NC3 thus obtained and not according tothe invention (whose platinum content is about 0.12% by weight) aregiven in Table 2.

Effect of Treatments of MFI by Ammonium Hexafluorosilicate, onSelectivities at Iso-Approach to Equilibrium

Table 1 describes the performance of the catalysts C1 and NC1 and Table2 describes that of the catalysts C2, C3, NC2 and NC3, preparedaccording to the modes of operation described hereinabove. The effect ofthe treatment with ammonium hexafluorosilicate on selectivities isparticularly apparent.

                  TABLE 1                                                         ______________________________________                                        Charges         80% (by weight) meta-xylene +                                    20% (by weight) ethylbenzene                                               Catalysts       NC1        C1                                                   Examples 4 1                                                                  % AEQ meta-xylene 95 95                                                       % yield C.sub.8 (aromatics + 68 82                                            naphthenes)                                                                   % Dismutation 17 8                                                            % Desalkylation 16 14                                                         % Cracking 7 3                                                                Ratio of 0.43 0.65                                                            para-xylene/meta-                                                             xylene                                                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Charges      80% (by weight) ortho-xylene +                                      20% (by weight) ethylbenzene                                               Catalysts    NC2    C2         NC3  C3                                          Examples 5 2 6 3                                                              % AEQ o-xylene 97 97 96.5 96.5                                                % Yield C.sub.8  67 80 70 79                                                  (aromatics +                                                                  naphthenes)                                                                   % Dismutation 14 6 16 9                                                       % Desalkylation 13 12 12 11                                                   % Cracking 4.5 1.5 2 1                                                      ______________________________________                                    

In Tables 1 and 2, the values for the spatial speeds (pph) in (hours)-1adjusted to obtain % AEQ of ortho-xylene and meta-xylene are comparable.

The catalyst C1 according to the invention exhibit higher performancethan the prior art NC1 catalyst NC1. In fact, at iso-approach toequilibrium of the meta-xylene, the rate of secondary dismutationreactions which leads, i.a., to the formation of trimethylbenzenes isgreatly reduced in instances where the catalyst is the catalyst C1according to the invention. On the other hand, it appears that thecatalyst treated according to the invention (C1) gives a cracking ratewhich is lower than the prior art catalyst NC1, but gives, inparticular, greater selectivity with respect to para-xylene isomer. As aresult, the yield of C₈ aromatics+naphthenes obtained with the catalystis greater than that of the prior art catalyst. The catalyst treatedaccording to the invention thus has the two-fold advantage that the rateof undesirable reactions is reduced (dismutation and crackingreactions), while the selectivity with respect to para-xylene formed isincreased.

The catalysts C2 and C3 according to the invention show higherperformance than the catalysts NC2 and NC3 of the prior art. In fact, atiso-approach to equilibrium of the o-xylene, the isomerization yield ofC₈ aromatics+naphthenes obtained on the catalysts C2 and C3 is greaterthan that of the catalysts NC2 and NC3. On the other hand, with the MFItreated according to the invention (catalysts C2, C3), the secondarydismutation reaction which results in the formation of trimethylbenzenesis greatly inhibited in comparison to that obtained in the presence ofnon selective MFI (catalysts C2 and C3). Likewise, the rate of thecracking reactions is greatly reduced on catalysts containing MFIzeolite which has been subjected to the treatment with at least oneaqueous solution of ammonium hexafluorosilicate according to theinvention. Moreover, it can be seen that the rate of the desalkylationreactions is not modified by the treatment carried out according to theinvention.

The X fluorescence analysis on the MFI; HMFI-1, HMFI-2 and HMFI-3treated according to the invention shows that the global (atomic) Si/Alratios thus measured are substantially identical to the global (atomic)Si/Al ratios of the initial MFI prior to treatment. On the other hand,an increase in the Si/Al ratios measured by XPS (ESCA) is noted afterthe treatment according to the invention.

What is claimed is:
 1. A process for the isomerization of a C₈ -aromaticcut, comprising subjecting said cut to effective conditions in thepresence of a catalyst comprising a matrix, an MFI zeolite and at leastone group VIII element, wherein crystals of the MFI zeolite aredealuminated on outer surfaces at least 37% by at least one treatmentwith at least one solution of a fluorosilicate having a cation which isNH₄ ⁺, an alkylammonium, K⁺, Na⁺, Li⁺, Ba²⁺, Mg²⁺, Cd²⁺, Cu⁺, Cu²⁺,Ca²⁺, Cs⁺, Fe²⁺, Co²⁺, Pb²⁺, Mn²⁺, Rb⁺, Ag⁺, Sr²⁺, Zn²⁺, Tl⁺, or H⁺, theoverall dealumination of said zeolite thus treated being less than 5percent on an atomic basis, the atomic Si/Al ratio of the MFI zeoliteprior to treatment with fluorosilicate being at least 5 and less than100.
 2. A process according to claim 1, wherein the MFI zeolite istreated with a an ammonium hexafluorosilicate solution.
 3. A processaccording claim 1, wherein the treatment is carried out between 20 and100° C. and at a pH of between 4 and
 8. 4. A process according claim 1,wherein the group VIII element is platinum or palladium.
 5. A processaccording claim 1, wherein the group VIII element is deposited on thezeolite.
 6. A process according claim 1, wherein the group VIII elementis deposited on the matrix.
 7. A process according to claim 1, whereinthe matrix is alumina, magnesium, silica alumina, natural clays, or amixture thereof.
 8. A process according to claim 1, conducted at atemperature between 240 and 600° C., a pressure is between 0.05 and 10MPa, a spatial speed is between 0.5 and 200 h⁻¹, and a H₂ /HC molarratio between 0.5 and
 12. 9. A process according to claim 8, in whichthe cut treated is a mixture of xylene.
 10. A process according to claim9, in which the cut treated also contains ethyl benzene.
 11. A processaccording to claim 1, wherein the catalyst is dealuminated on its outersurface at least 37-61%.
 12. A process according to claim 1, wherein thecatalyst is dealuminated on its outer surface at least 52-61%.
 13. Aprocess for the isomerization of a C₈ -aromatic cut, comprisingsubjecting said cut to isomerization conditions in the presence of acatalyst comprising a matrix, a MFI zeolite and at least one group VIIIelement, wherein the MFI zeolite is dealuminated on its outer surface atleast 37% on an atomic basis and dealuminated overall less than 5% on anatomic basis, said dealumination being effected by at least onetreatment with at least one solution of a fluorosilicate having a cationwhich is NH₄ ⁺, an alkylammonium, K⁺, Na⁺, Li⁺, Ba²⁺, Mg²⁺, Cd²⁺, Cu⁺,Cu²⁺, Ca²⁺, Cs⁺, Fe²⁺, Co²⁺, Pb²⁺, Mn²⁺, Rb⁺, Ag⁺, Sr²⁺, Zn²⁺, Tl⁺, orH⁺.
 14. A catalyst according to claim 13, in which the MFI zeolite priorto dealumination has an atomic Si/Al ratio of between 5- less than 100.15. A process according to claim 13, wherein the catalyst isdealuminated on its outer surface at least 37-61%.
 16. A processaccording to claim 13, wherein the catalyst is dealuminated on its outersurface at least 52-61%.